The present invention relates to abrasive cleaning of a workpiece with high velocity sublimable particles and to production of such particles.
It is commonly known to blast a workpiece with a particulate abrasive that either melts or sublimes at room temperature for cleanly dissipating the abrasive subsequent to its use, thereby avoiding contamination of the workpiece or its environment. The abrasive can be frozen water, typically called "ice", solid carbon dioxide, typically called "dry ice", or combinations comprising one or both of these materials. Dry ice particles are produced in a variety of ways as discussed below, abrasive cleaning most typically employing extruded pellets. Common problems associated with dry ice blasting include the following:
1. The size of the particles greatly affects blasting quality and efficiency, small particles being desired for reaching small features of the workpiece, for avoiding damage to delicate workpiece surfaces, and because small particles are easier to accelerate; PA1 2. It is more difficult to make small particles than big ones; PA1 3. The particles are subject to degradation by subliming, by melting, and by abrasion or pulverization during transport to the workpiece, these mechanisms having increasingly adverse effects as the particle size is reduced; and PA1 4. The particles are subject to clogging in storage and transport to a blasting nozzle. PA1 1. There is no real-time control of flow from the granulator; PA1 2. They are complex and expensive because sifting and/or sizing is required following crushing for promoting flowability; PA1 3. They are further complex, expensive and unreliable in that they require agitators, fluidizers and/or special cooling, insulating, moisture barriers and the like because stored granules are particularly subject agglomeration and sublimation; and PA1 4. Metering of stored granules is particularly difficult in the presence of agglomeration. PA1 (a) providing a rotatable cutter having a cutting plane; PA1 (b) moving a supply of dry ice along a feed path into the cutting plane; PA1 (c) abrading a surface of the supply of dry ice to continually form dry ice granules of a predetermined maximum size and size distribution; PA1 (d) conducting the granules to an exit duct; and PA1 (e) accelerating the granules into a high velocity stream for blasting a workpiece.
Early equipment for dry ice blasting employed pelletizers that were developed for the food processing industry, having a high production rate capability. These devices, being disclosed for example in U.S. Pat. Nos. 4,038,786 to Fong and 4,744,181 to Moore, operate cyclicly, with intermediate storage of the pellets, and metering at a desired rate only when blasting. Cyclic pelletizer blasting machines are large, heavy, and complex, requiring a liquid CO.sub.2 storage vessel, a cryogenic pump, and insulated supply and return piping, being thus limited in use to fixed facility, non-portable cleaning. Also they are expensive, costing from about $100,000 to $250,000, and power-hungry, requiring from 1.2 to 1.5 horsepower per pound of dry ice per minute. Further, dry ice pellets are difficult to store for even small periods without agglomeration which is detrimental to blasting. Moreover, the pellets are subject to sublimation unless special cooling is provided. The smaller the pellets the more susceptible they are to clumping and/or sublimation.
A later development is continuous pelletizer blasters as disclosed, for example, in U.S. Pat. No. 4,389,820 to Fong et al, wherein liquid CO.sub.2 is dispensed and frozen in a snow chamber, the snow falling into a planetary extruder die mechanism where it is compacted into pellets by being forced through radial holes of a ring-shaped die, the length of the pellets being defined by structure that fractures the material by partially blocking the exit paths from the die. The pellets are produced as needed on a real-time basis, with metering by adjusting the rate of pelletizing, for significant savings in the size and weight of the machines. The continuous pelletizer blasters of the prior art require intensively engineered and expensive pelletizers, the overall cost being in the same range as the cyclic blasters. Also, the dynamic range of production is limited such that typical units having sufficiently low minimum output can adequately feed only a single blast cleaning gun or nozzle. Further, they are as power hungry as the cyclic pelletizers, and the power source must be able to handle the dynamic range of pellet production rates. Moreover, portability is limited because of a need for a supply line to a liquid CO.sub.2 source, together with the other liquid CO.sub.2 handling facilities of the cyclic blasters.
More recently there have appeared portable blasters that are filled from a remote source of the pellets, such as are disclosed in U.S. Pat. Nos. 5,071,289 and 5,288,028 to Spivak, and 5,203,794 to Stratford. These machines can be moved easily, being untethered by liquid CO.sub.2 lines and they are much smaller and lighter than ones having pelletizers. They are also less expensive, costing approximately $25,000 to $50,000. However, they can be used only where there is access to a supply of pellets, typically in only a few large metropolitan areas. Long continuous or automated operation is not possible in that they must be refilled with pellets every 30 to 180 minutes. Further, they are particularly susceptible to agglomeration and/or sublimation of the pellets. Moreover, they require a metering device that is difficult to provide in that metering should be smooth and variable for different applications, even in the presence of some agglomeration.
Another form of dry ice blasting equipment is snow guns that provide particles of limited mass and hardness, for cleaning delicate surfaces. Such equipment is not capable of aggressive cleaning, and still requires liquid CO.sub.2 facilities and fluid additives. A further form of the blasting equipment is granulator blasters, such as disclosed, for example, in U.S. Pat. Nos. 4,707,951 to Gibot and 4,965,968 to Kelsall. These machines typically crush pieces or chunks of dry ice in a batch mode at a relatively high rate, the crushed dry ice being stored in a hopper and metered as required for blasting. The granulator blasters of the prior art are subject to one or more of the following disadvantages:
Thus there is a need for a particulate formation and blasting system that effectively and reliably produces and delivers uniformly small particulate on demand and without clogging or degradation of the particles.